Cell-cell signaling controls many processes in the biological world, including development, pathogenesis, growth, mating, and transformation. Signaling processes are often mediated by factors (e.g. hormones, pheromones, neurotransmitters) that are produced by some cells and sensed by others. In bacteria, the ability to sense and respond to high population density is a type of cell-cell signaling often referred to as quorum-sensing. Cells produce extracellular signaling molecules that accumulate as population density increases, and a physiological response occurs at a critical density. The long-term goal of this project is to understand how Bacillus subtilis modulates gene expression and development in response to environmental conditions, with particular focus on aspects of peptide and cell-cell signaling. The major pathway for quorum sensing in B. subtilis involves activation of the transcription factor ComA, a response regulator that is active when phosphorylated. The activity of ComA is modulated by at least two different peptide signaling molecules that accumulate in culture supernatant as cells grow to high population density. A major challenge is to elucidate the range of cellular processes that are controlled by cell-cell signaling in a single species. This includes characterizing the genes that are regulated in response to population density, identifying the signaling molecules and pathways, and characterizing the web of overlapping interactions between responses to population density and other physiological signals. We will investigate these issues by characterizing the ComA-dependent quorum response, by characterizing other genes that are likely to be involved in cell-cell signaling, and by testing directly for and characterizing additional cell density-regulated responses. Central to this project is the use of DNA microarrays to characterize mRNA levels under a variety of conditions and in a variety of mutants. Our studies on quorum sensing and gene expression in B. subtilis, are relatively simple, experimentally accessible microbe should provide insights into general mechanisms of cell-cell signaling, signal transduction, and regulation.